As is well-known, gases such as methane, carbon dioxide, acetylene, and ammonia have an absorption band absorbing light of a specific wavelength in accordance with the rotation of constituent molecules of the respective gases or vibration between constituent atoms.
For example, in the case of methane gas, the gas has absorption bands absorbing light of specific wavelengths such as a 1.65 μm band, a 3.3 μm band, and a 7.7 μm band.
FIG. 12 is a graph showing light absorption spectrum characteristics of methane gas at a wavelength of 1.65 μm.
Namely, as shown in FIG. 12, in accordance with the light absorption spectrum characteristics of methane gas, it can be understood that the light intensity at a wavelength of 1.65 μm is damped in a dip-shape.
Further, a laser absorption spectroscopy type gas detecting device applies a laser beam to a gas generating place from a remote position by using such light absorption characteristics of the gas, and detects the existence of gas optically.
By using such a laser absorption spectroscopy type gas detecting device, gas leakage of, for example, city gas, at a chemical plant or the like can be detected from far away.
It should be noted that, generally, such a laser absorption spectroscopy type gas detecting device has been required to aim for compactness and a lighterweight as a portable type, so as to be easily carried into a site at the time of a gas leak or at the time of inspection.
FIG. 13 is a side sectional view showing a structure of a conventional laser absorption spectroscopy type gas detecting device 50 disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-103887.
Namely, as shown in FIG. 13, in the laser absorption spectroscopy type gas detecting device 50, a convex-shaped condenser lens 52 is provided at a front surface of a housing 51.
At the central portion of the condenser lens 52, a light source portion 53 formed from a laser diode (LD) module is provided.
At a remote position of a predetermined distance, a laser beam of a predetermined wavelength band is emitted, from the light source portion 53, into a space at which it is supposed that gas to be detected exists.
Then, the laser beam emitted from the light source portion 53 is reflected by a material body having the property of reflecting light, such as the wall of a building existing within the distance which the external laser beam can reach, and returns to the laser absorption spectroscopy type gas detecting device 50.
After this reflected light is condensed by the condenser lens 52 of the laser absorption spectroscopy type gas detecting device 50, the light is received by a light receiver 54 provided at the inner portion of the housing 51.
Here, if gas to be detected exists at the front portion of the external reflecting material body, because the laser beam emitted from the light source section 53 passes through the gas to be detected, the light component of a specific wavelength of the laser beam is absorbed by the gas to be detected.
Therefore, in the reflected light, the light intensity at the light component of this specific wavelength is damped by the light absorption characteristics of the gas to be detected.
Accordingly, the light receiving level at the light component of this specific wavelength at the light receiver 54 is also damped.
On the other hand, when gas to be detected does not exist at the front portion of the reflecting material body, damping of the light intensity at the light component of the specific wavelength by the light absorption characteristics of the gas to be detected does not occur. Therefore, damping of the light receiving level at the light component of this specific wavelength at the light receiver 54 does not also occur.
In this way, the light receiver 54 outputs an electric signal for detecting whether gas to be detected exists at the front portion of the reflecting material body, in accordance with the damping degree of the light component having a wavelength corresponding to the light absorption spectrum of the gas to be detected.
Further, a signal processing portion (not shown) carries out signal processing for detecting the presence/absence of the gas to be detected, on the basis of the electric signal corresponding to the light-receiving state from the light receiver 54.
However, in the conventional laser absorption spectroscopy type gas detecting device 50 as described above, the condenser lens 52 used in the device is large (for example, the diameter is 12 cm, the thickness is 3 cm, and the weight is 1 kg or more).
Therefore, the housing 51, holding the condenser lens 52 in the state of preventing optical axis offset or the like, requires rigidity strength of that extent, and the weight of the housing 51 itself becomes heavy.
In accordance therewith, because the entire device of the conventional laser absorption spectroscopy type gas detecting device 50 is large and heavy, there are the problems that transport thereof cannot be easily carried out and the device is unsuitable for portable use.
Further, in the conventional laser absorption spectroscopy type gas detecting device 50 as described above, the light source section 53 of a predetermined size (for example, the diameter is 4 cm) is provided at the central portion including the optical axis of the condenser lens 52. Therefore, the effective condensing area of the condenser lens 52 decreases, and there is a problem that the condensing efficiency tends to deteriorate.
Due to such an arrangement of the light source section 53, in order to ensure the effective condensing area of the condenser lens 52, it is unavoidable that the condenser lens 52 itself is large.
Further, the conventional laser absorption spectroscopy type gas detecting device 50 as described above has a structure in which the gas to be detected is detected in a range of distances (measuring distances) to the place which is apart by about 50 m from the closest to the device.
Therefore, the focal distance of the condenser lens 52 requires a predetermined length in accordance with the measuring distance.
In accordance therewith, in the conventional laser absorption spectroscopy type gas detecting device 50 as described above, because the length from the condenser lens 52 to the light receiver 54 must be long, it is unavoidable that the housing 51 becomes large and heavy.
Here, if a short focal lens is used as the condenser lens 52 in order to make the housing 51 compact, due to the condenser lens 52 being thicker, the condenser lens 52 becomes heavier.
On the other hand, if a long focal lens is used as the condenser lens 52, the length of the housing 51 becomes long and heavy.
FIG. 14 is a side sectional view showing another structure of the conventional laser absorption spectroscopy type gas detecting device 50 disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-103887.
In FIG. 14, structural portions which are the same as those of the laser absorption spectroscopy type gas detecting device 50 of FIG. 13 are denoted by the same reference numerals, and description thereof will be omitted.
Namely, in the conventional laser absorption spectroscopy type gas detecting device 50 as shown in FIG. 14, a focal point adjustment mechanism 60 is provided within the housing 51.
As described above, the laser absorption spectroscopy type gas detecting device 50 detects the presence/absence of gas, without contact, with a predetermined distance range.
In accordance with a change of the measuring distance, the image formation position of the reflected light condensed by the condenser lens 52 changes.
Here, the focal point adjustment mechanism 60 detects the image formation position of the reflected light, and carries out focal point adjustment such that the light receiver 54 is positioned on the image formation position.
Namely, in this focal point adjustment mechanism 60, after one portion of the reflected light condensed by the condenser lens 52 is branched off by a half mirror 61, the light is irradiated to an image sensor 63 via a lens 62.
The branched-off light detected on the image sensor 63 moves in accordance with the focal point position of the light receiver 54.
Here, due to moving means 64 being controlled in accordance with the detected position at the image sensor 63, the light receiver 54 is moved in an optical axis direction.
The focal adjustment mechanism 60 thereby can carry out focal point adjustment such that the image formation position of the reflected light condensed by the condenser lens 52 is positioned on the light receiving surface of the light receiver 54.
By the way, in the conventional laser absorption spectroscopy type gas detecting device 50 as described above, because the laser beam emitted toward the gas to be detected is invisible light, there is the problem that the user cannot easily confirm by visual observation what position the laser beam is irradiated to.
Therefore, even if the conventional laser absorption spectroscopy type gas detecting device 50 has the focal point adjustment mechanism 60 as described above, there has been the problem that the gas to be detected cannot be reliably detected if the irradiated position of the laser beam is unclear.
Here, a laser absorption spectroscopy type gas detecting device, which is configured such that a user confirms an irradiated position of laser beam by visual observation due to a laser pointer emitting visible laser beam being provided, is known.
However, in such a laser absorption spectroscopy type gas detecting device, the device as the laser pointer must be separately installed, and further there are the problems that the weight becomes heavier and costs increase.
Note that this laser pointer is for a user to confirm an irradiated position of laser beam on a reflecting material body by visual observation, and focal point adjustment within the device cannot be immediately carried out.
Further, the focal point adjustment mechanism 60 has a large number of parts and needs an arithmetic processing portion for adjusting the focal point position. Therefore, there is the problem that the structure is complex and reducing the cost for the entire device cannot be attempted.